1. Field of the Invention
The present invention relates to a high power semiconductor laser device, more particularly, which has superb optical output efficiency due to decreased internal optical loss.
2. Description of the Related Art
Recently, a high power semiconductor laser device has found its various applications, for example, in an industrial field such as optical telecommunication, optical recording devices, welding and cutting and in a medical field. The high power semiconductor laser device is also being used as a visible ray laser for displays, with a prospect for growth in its application in the future.
In general, the semiconductor laser device can have high output characteristics by virtue of two methods. That is, firstly a beam emission portion of laser diodes can be designed in a large size, or secondly the laser diodes are arranged in a long bar shape and the laser bars can be stacked.
The former method concerns enhancing output of the laser diodes themselves. Here, the beam emission portion, i.e., an emitter can be designed with a larger width (“W” of FIG. 1) or a radiator itself can be prolonged to a greater length (“L” of FIG. 1). The emitter with a larger width beneficially achieves high output but may distort beam shape. On the other hand, the resonator with a greater length distorts beam shape less, and lowers current density to restrain temperature rise of the device, thereby enhancing output. However, the resonator with a greater length L disadvantageously degrades optical output efficiency due to internal optical loss [cm−1] of the resonator. Therefore, a key in designing the high power semiconductor laser is to reduce internal optical loss of the resonator so that decrease in the efficiency of current-light characteristics can be minimized even if the resonator is prolonged in its length to yield high output.
This internal optical loss of the resonator will be explained with reference to FIG. 1 which illustrates a conventional high power semiconductor laser device.
Referring to FIG. 1, the high power semiconductor laser device 10 includes first and second conductivity type clad layers 12 and 15 formed on a conductive substrate 11 and an active layer 14 interposed therebetween. A first optical guide layer 13a is disposed between the first conductivity type clad layer 12 and the active layer 14, and a second optical guide layer 13b is disposed between the second conductivity type clad layer 15 and the active layer 14. A second conductivity type contact layer 16 is disposed on the second conductivity type clad layer 15. A current blocking layer 17 made of an insulating material is disposed on the second conductivity type contact layer 16 to confine a current injection region. This accordingly defines the region of a resonator R. Also, a first electrode 19a is formed on an underside surface of the semiconductor laser device 10 and a second electrode 19b is formed on a top surface thereof.
The first and second optical guide layers 13a and 13b are undoped layers having higher reflectivity than the first and second conductivity type clad layers 12 and 15. The first and second optical guide layers 13a and 13b serve to confine laser beam shape while the active layer 14 confines movement of electrons and holes. Thus, the first and second optical guide layers 13a and 13b are referred to as a separate confinement heterostructure (SCH) layer. The optical guide layers (or SCH layer) can have a great thickness of about 500 nm to 2 μm, thereby featuring a large optical cavity (LOC) structure. This structure reduces the overlap between the optical mode and the first and second conductivity type clad layers 12 and 15 to lower internal optical loss.
However, in a case where the optical guide layers are increased in thickness to decrease internal optical loss, it may trigger a higher order mode beam, seriously distorting beam shape and causing mismatch with an optical system. This poses difficulty to practical application thereof.
Accordingly, the optical guide layers can be limitedly increased in their thickness. Thus, as shown in FIG. 2, this leads to the beam mode overlapping the first and second conductivity type clad layers 12 and 15, thereby hardly decreasing internal optical loss sufficiently.
Consequently, in the conventional semiconductor laser device, the resonator is limitedly prolonged in its length to enhance output owing to such internal optical loss problems.